Vacuum drums in the form of a rotary member having vacuum suction holes opening onto a cylindrical surface for the support of flexible sheet media peripherally thereof are known for various applications. Paper processing and printing machines, for example, utilize such rotary vacuum drums to convey sheets from one part of the machine to another. Other machines, such as laser printers and rotary data scanning and recording devices, use similar drums to support a flexible data plate medium for interaction with a printing or recording head.
It is a common problem in such devices, especially in large drum devices, to experience local distortion of the retained medium caused by deformation of the drum surface due to vacuum draw through the cylindrical shell into an internal chamber. Another problem, where the same drum is used to handle various sized media is the loss of vacuum force at uncovered holes.
Various approaches have been taken for supporting flexible sheet material of different sizes on a vacuum drum surface. Those employing means to close uncovered holes when smaller sized sheets are drawn to the surface, tend to be complicated structures with elaborate mechanical channeling movements, such as shown in U.S. Pat. Nos. 4,145,040 and 4,202,542.